Civil Engineering Reference
In-Depth Information
7
Design of Flexural Steel
Members
7.1 INTRODUCTION
Members designed to carry primarily bending forces or flexure are typically found in
steel railway bridges as girders, beams, floorbeams, and stringers. These beam and
girder members experience normal tensile, normal compressive and shear stresses
and are designed considering strength and serviceability criteria.
Flexural members must be designed in accordance with theAREMA (2008) allow-
able stress method to resist normal tensile stresses based on yield strength at the net
section. Flexural members subjected to cyclical or fluctuating normal tensile stress
ranges must be designed with due consideration of stress concentration effects and
metal fatigue. The members must also be designed for strength to resist shear and
compressive normal stresses with due consideration of stability.
Flexural member serviceability design for stiffness is achieved by respecting live
load deflection limits for simple spans used in freight rail operations. Lateral rigidity
to resist vibration and other effects (lateral loads from track misalignments, rail wear,
track fastener deterioration) is generally provided by designing for the lateral forces
recommended by AREMA (2008).
Steel beams and girders for railway spans can be of noncomposite material (steel,
timber, or independent concrete deck) or composite materials (integral concrete deck)
design. Noncomposite beams and girders are used for the design of both open and
ballasted deck spans. Composite design provides a ballasted deck span. The relative
merits of each system are discussed in Chapter 3.
7.2 STRENGTH DESIGN OF NONCOMPOSITE
FLEXURAL MEMBERS
Steel girders, beams, floorbeams, and stringers are designed as noncomposite flexural
members. They must be designed for internal normal flexural and shear stresses
created by combinations of external actions (see Table 4.5).
7.2.1 B
ENDING OF
L
ATERALLY
S
UPPORTED
B
EAMS AND
G
IRDERS
Elastic strains in beams and girders with at least one axis of symmetry that undergo
bending with small deformations, and where plane sections through the beam
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